DESCRIPTION: (Applicant's abstract) The goal of the proposed research is to understand how neuronal cell fate is determined in vertebrate organisms by the action of NeuroD, an embryonic basic helix-loop-helix transcription factor that the principal investigator discovered. Transcription factors containing a basic helix-loop-helix (bHLH) motif play important roles in specifying cell fates of many tissue types in both invertebrate and vertebrate organisms. The HLH domain mediates formation of homo or heterodimers among bHLH proteins while the basic domains of the resulting dimer complex specify the E-box (CANNTG) DNA recognition site. For example, during mammalian skeletal myogenesis, tissue specific bHLH proteins, such as MyoD, and ubiquitously expressed bHLH proteins, such as E proteins, form a transcriptional complex which activates the skeletal myogenic program. When MyoD is overexpressed in non-muscle cells, it can convert the host cells into muscle cells. The principal investigator has recently cloned a new bHLH gene which is named neuroD. We have shown that neuroD is expressed in differentiating neurons in developing mouse and frog embryos. When mis-expressed in developing frog embryos by RNA injection at the two cell stage, neuroD converts embryonic ectoderm, such as epidermis, into neurons. In addition, ectopic NeuroD causes neural precursor cells to prematurely differentiate into neurons. Based on these observations, the principal investigator hypothesizes that NeuroD is a differentiation factor specific to the neuronal cell lineage. The proposed research provides a further analysis of the molecular mechanisms of NeuroD during vertebrate neurogenesis. Two different approaches will be taken: 1) Determination of the normal function of NeuroD during mouse development by gene targeting; ii) Elucidation of the molecular mechanisms by which ectopically expressed NeuroD converts non-neural cells in Xenopus embryos into fully differentiated neurons. Understanding how neuroD generates ectopic neurons and causes premature differentiation could have significant implications in the medical field. First, understanding the role of NeuroD as a differentiation factor and elucidating how it overrides the proliferative signals of mitotic cells could prove invaluable in approaches to treating cancer. For example, it may help to understand whether the molecular basis of certain brain tumors is a failure to correctly differentiate or to maintain the differentiated state of neuronal cells. Second, the ability for neuroD to convert ectodennal cells into fully differentiated neurons may provide the first step for gene therapy strategies aimed at generating neurons to patients with neurodegenerative diseases and nerve injuries.